Contrasting Trophic Niche and Resource Use Dynamics Across Multiple American Horseshoe Crab (Limulus polyphemus) Populations and Age Groups

Abstract

Horseshoe crabs (Limulus polyphemus) exhibit ontogenetic and geographic variability in migratory patterns, yet the implications of movement on their resource use dynamics remain poorly understood. Here, we evaluate horseshoe crab resource use across ontogeny (instars 14–18 and adults), sex, seasons, and between two spatially distinct populations that exhibit different migratory patterns. Multi-tissue stable isotope analyses of carbon, nitrogen, and sulfur were used to examine (1) ontogenetic and sex-specific variability in the reliance of resource pools; (2) variation in isotopic niche breadth within and among populations; and (3) seasonal variability in resource use within juvenile life stages. We found evidence for subtle ontogenetic shifts in resource use, with increased contributions of phytoplankton with ontogeny, but all age groups predominantly relied on sedimentary organic matter (SOM) pools. Sex-specific differences in resource use were observed with SOM serving as the predominant source assimilated in males, while females exhibited a higher dependence on phytoplankton resource pools, indicating foraging or spatial preferences may be different between sexes. Low trophic niche overlap between adult and juvenile horseshoe crabs was evident, likely arising from differences in migratory patterns and size constraints. Within juveniles, no seasonal differences in resource use were observed, indicating that dietary patterns may remain static across temporal scales, consistent with limited dispersal of that life stage. Spatial differences in resource use were, however, observed between adult crabs likely reflecting the migratory strategies of different populations. Our results are consistent with previous evidence that horseshoe crabs are dietary generalists but provide novel insights into the linkages between movement and trophic patterns.

Effects of sodium heparin on δ13C, δ15N and δ34S values in avian whole blood

“…However, our results differed from those of Lemons et al (2012) with respect to δ 15 N. Lemons et al (2012) found that whole blood samples treated with sodium heparin were significantly enriched in 15 N relative to untreated samples whereas our study found no significant difference between δ 15 N values in treated and untreated samples. Lemons et al (2012) did not assess δ 34 S, but our findings with respect to δ 34 S values differed from those of Bopp et al (2022), who found no significant difference in δ 34 S between treated and untreated samples of Atlantic Horseshoe Crab blood. These differences in findings across the literature highlight that isotopic factors can vary depending on the taxa (2007), the addition of the anticoagulant EDTA to avian whole blood samples altered δ 13 C and δ 15 N values by 0.57‰ and 0.63‰, respectively, though this effect was considered negligible relative to field signatures.…”

“…For each isotope, denoted as the jth group, and individual, denoted as the ith group, we computed the difference (D) between the treatment and control sample as: D ji = X treatment − X control , where X represents δ 13 C, δ 15 N or δ 34 S and where D ji ~N(μ j , σ 2 ), denoting that D is drawn from a normal distribution with mean μ j and variance σ 2 . We used a semi-informative prior for μ (μ j ~N(0,1)) with prior information coming from Lemons et al (2012) and Bopp et al (2022), who found that differences between treatment and control groups were between 0 and 1‰ for whole blood. We chose a semi-informative prior, rather than an uninformative prior, as the incorporation of prior information into Bayesian models increases the precision of parameter estimations (Banner et al 2020).…”

“…In contrast, a study of stable isotopes in shark blood demonstrated that red blood cells treated with sodium heparin were significantly enriched in 15 N relative to control samples (Weideli et al 2019). Bopp et al (2022) found that sodium heparin did not significantly impact δ 34 S values in Atlantic Horseshoe Crab Limulus polyphemus blood. This contrast in literature along with the notion that aspects of isotope ecology, such as trophic enrichment factors, are known to vary with taxa (Caut et al 2009) highlight that it is important to understand the effects of anticoagulants on the specific taxa of interest rather than applying results across studies of other taxa.…”

“…(2012) and Bopp et al . (2022), who found that differences between treatment and control groups were between 0 and 1‰ for whole blood. We chose a semi‐informative prior, rather than an uninformative prior, as the incorporation of prior information into Bayesian models increases the precision of parameter estimations (Banner et al .…”

“…Bopp et al . (2022) found that sodium heparin did not significantly impact δ 34 S values in Atlantic Horseshoe Crab Limulus polyphemus blood. This contrast in literature along with the notion that aspects of isotope ecology, such as trophic enrichment factors, are known to vary with taxa (Caut et al .…”